Alert (backend)

[polymorpher]

Following the introduction of user accounts in #190 (optional to users) and the synchronization feature in #255, another major benefit the user would get (by providing their real email) is they can receive alerts for critical operations occurred on their wallets.

For example, the user may want to receive an alert when funds leave their wallets, or alerts marked with high-priority, if the daily spending limit of one of their their wallets is raised (or decreased), or in case of staking, when their stakes are undelegated. Timely alerts could give the user ample time to prevent hacking, or to stop/mitigate unintended uses of the wallets (e.g. sent more money than they intended)

For implementation, we could start with a simple backend with SendGrid enabled, that continuously monitor events emitted from wallet addresses that enabled the alert service. Whenever an event that meets the user-selected criteria is triggered, an email should be sent to the user. Users should be able set their own criteria after username/password authentication using the frontend UI. The UI component will be discussed in a separate issue.

[polymorpher]

User system implemented in #362